In general, when using a numerical control machine tool to machine a workpiece, the machining program is set with a plurality of tool paths using data of machining command points including coordinate components (machining point data). At this time, if there are differences in machine point data in the normal direction of the machined surface between adjoining tool paths, the machined surface will be formed with step differences and the machined shape of the workpiece will be degraded. In order to suppress such step differences between tool paths, an apparatus which inserts auxiliary points between consecutive machining command points (machining points) of one tool path (below, called the “first tool path”) by using the tool paths at the two sides of this tool path (below, called the “second and third tool paths”) and which uses an approximation curve of these consecutive machining points and auxiliary points to generate a tool path is known (for example, see Patent Literature 1).
The apparatus described in this Patent Literature 1 determines center points of consecutive machining points on the first tool path, draws vertical lines from the center points to the second and third tool path, and determines machining points corresponding to the consecutive machining points on the first tool path, on the second and third tool paths. It uses the machining points on these tool paths to calculate approximation curves of the tool paths, and derives auxiliary points based on these approximation curves and the positions of the legs of the vertical lines drawn from the center points of the first tool path.
However, when, like in the apparatus which is described in the above Patent Literature 1, calculating approximation curves of a plurality of the tool paths to derive auxiliary points, the processing is troublesome and it takes time to generate a tool path.